The present invention relates to a monolithic, fully dense, silicon carbide mirror or mirror substrate and methods of manufacturing. Applicant is unaware of any prior art teaching a ceramic material and/or method of manufacturing as taught herein nor use of such a material as a mirror.
The following prior art is known to Applicant:
U.S. Pat. No. 3,592,942 to Hauck et al. discloses a hard ceramic material described as "high temperature fabricated polycrystalline alumina, silicon carbide or boron carbide". The present invention differs from the teachings of Hauck et al. as contemplating a particular grain size, density, Weibull modulus, environment of use and structural mode of crack propagation and resistance thereto nowhere taught or suggested by Hauck et al.
U.S. Pat. Nos. 3,765,300 and 3,796,564 to Taylor et al. teach a dense carbide composite for various applications. Taylor et al. disclose that ". . . it is essential that the granular boron carbide in the initial mixture have a maximum particle size of about 300 microns or less, although coarser material may be employed to make composite bodies useful for less demanding purposes." Taylor et al. further disclose that "The modulus of rupture may be as low as 10,000 psi (700 kg/cm.sup.2), especially where granular boron carbide with a particle size greater than about 300 microns is included in the mix . . . ". As such, Taylor et al. fail to contemplate the grain size disclosed herein nor the method of manufacturing nor the intergranular fracture and resistance thereto, nor the contemplated environment of use disclosed herein.
U.S. Pat. No. 3,977,294 to Jahn discloses composite laminate material and method. This patent contemplates a composite laminate material including layers of graphite and ceramic materials adhered together through the use of an adhesive. The ceramic materials and method and applications disclosed herein are nowhere taught or suggested by Jahn.
U.S. Pat. No. 4,692,418 to Boecker et al. discloses a sintered silicon carbide/carbon composite ceramic body having fine microstructure. This patent discloses making of a silicon carbide material and, thereafter, infiltrating a source of carbon therein to provide the composite body. The present invention differs from the teachings of Boecker et al. as contemplating a monolithic silicon carbide material possessing unique properties and made in a manner not contemplated by Boecker et al.
U.S. Pat. No. 4,693,988 to Boecker et al. discloses a single phase silicon carbide refractory. The Boecker et al. material consists of a pressureless sintered product made from starting materials wherein a coarse fraction as well as a fine fraction are present. Boecker et al. disclose that the coarse fraction has a particle size ranging between 210 and 340 micrometers, huge particles as compared to those contemplated herein. In Boecker et al., all particles are carried through to the final microstructure, that is, they do not reduce in size during the sintering process. Test results show that such grains incorporated within an SiC microstructure would clearly exhibit transgranular fracture when impacted as compared to the present invention wherein intergranular fracture would occur but mechanisms to resist such fracture exist. The inventive grain size (material microstructure) disclosed in this patent application is preferably equal to or less than 7 micrometers which is required to facilitate intergranular fracture. That is, experimentation has revealed that silicon carbide grains larger than 7 micrometers generally exhibit transgranular fracture and grains smaller than 7 micrometers generally exhibit intergranular fracture. As such, the present invention clearly differs from the teachings of Boecker et al.
U.S. Pat. No. 4,876,941 to Barnes et al. discloses a ceramic composite comprising titanium boride combined with aluminum nitride. Hot pressing techniques are employed in the manufacture of this material. The present invention differs from the teachings of Barnes et al. as contemplating a monolithic silicon carbide evidencing a mode of failure defined as intergranular in nature. While aluminum nitride is employed in the process of manufacturing the inventive ceramic, it is only used as a densification aid and in proportion much smaller than the proportion of silicon carbide which is employed. In a composite, such as that which is disclosed by Barnes et al., the aluminum nitride is an integral part of the microstructure, that is, aluminum nitride grains are present and can be specifically identified as aluminum nitride. In contrast to this, in the present invention, aluminum nitride behaves as a densification aid. After processing, one may not identify specific aluminum nitride grains. Aluminum nitride is not an integral part of the microstructure and, chemically, the finished ceramic body does not show the presence of aluminum nitride.
K. Nakamura and K. Maeda, in Silicon Carbide Ceramics, Volume 2, Edited by S. Somiya and Y. Inomata, Elsevier Applied Science, 1991, disclose hot-pressed SiC ceramics. These investigators have demonstrated a hot-pressed silicon carbide material using aluminum nitride (AlN) as a processing aid with a Weibull modulus of 13.8. The inventive silicon carbide material disclosed herein contemplates a Weibull modulus within a range of 18 to 30, much higher than Nakamura et al. which makes it possible to produce components with outstanding performance characteristics and exceptional reliability where prior state-of-the-art materials could not be used. The present invention also differs from the teachings of Nakamura et al. as contemplating densification techniques, relation between weight of AlN addition to SiC powder surface area and end use nowhere taught or suggested therein.